Devices that filter selected wavelengths of light are well known and have been in use for many years. Typically, a source of white light or light comprising components from wavelengths throughout the visible spectrum is filtered so as to transmit only the desired wavelengths. Among the various filters that are conventionally available for use as a light filter, absorption filters and interference filters are in common use.
Absorption filters are typically formed by imbuing a substrate with a dye that functions to absorb a given subset of visible light wavelengths. The result is that a portion of the light that is incident upon the absorption filter will be absorbed, and most of the remaining light will be transmitted through the filter. While effective, absorption filters are not ideally suited for many applications because the light that is absorbed is converted to heat, which tends to shorten the life of the dye, the filter substrate, or both. Absorption filters used in theatrical lighting, which are also known as “filter gels” or “gels”, for example, are typically used for only a relatively short period of time before the intensity of the light being filtered results in the unacceptable degradation of the absorption filter.
Interference filters that filter visible light, which are also known as dichroic filters, utilize very thin, alternating layers of a material with a high refractive index and a material with a low refractive index deposited on a substrate such as a rigid glass substrate. These filters have little absorbance and transmit some wavelengths, while reflecting other wavelengths and causing destructive interference of out of phase wavelengths. This results in a different apparent color on each side of the filter (i.e. dichroic). Because dichroic filters do not absorb a significant amount of light incident upon them, they do not suffer from the same heat issues as absorption filters. Further, the deposited layers of materials typically resist physical degradation better than dyes, which often leads to longer and better performance. Unfortunately, many conventional dichroic filters are formed on glass substrates, which makes the filters relatively heavy, subject to breakage, and relatively costly to produce and distribute compared to gel filters.
What are needed in the art are improved optical filters that can be used to filter light without degrading due to heat buildup or dye instability, and that also can be produced, handled, and/or distributed more easily and safely than glass based filters.